While the top seeds in the women's draw at the U.S. Open continue to fall, Serena Williams just hovers over all of them.

Williams, the No. 1 seed and defending champion, is rolling toward a third consecutive Open title, an 18th Grand Slam overall. Her 6-3, 6-3 win over Varvara Lepchenko Saturday was a little more work than the score line would indicate, but there was little doubt in the outcome.

Her next opponent is the unseeded Kaia Kanepi, who has not beaten Williams in three previous matches, the last one being five years ago.

It's possible she could meet her only serious challenger in the top half of the draw, Victoria Azarenka, who beat Elena Vesnina in two sets. Azarenka, who has lost the last two Open finals to Williams, is gaining some momentum after an injury-ravaged season.

She hasn't made it past the fourth round in any Slam this year, losing to Ivanovic in Australia, Garbine Muguruza -- of all players -- at the French, and Alize Cornet at Wimbledon. She has won five titles this season, including two of the three hard court tournaments she played before the Open.

In typical deadpan, Williams addressed her season bereft of major titles. "I've been a casualty this whole year at Grand Slams," she said. "I'm just hoping to keep staying in there."

While not nearly as powerful as Williams, the lefthanded Lepchenko, who practices often with Williams, demonstrated a surprising serve that kept Williams guessing. Lepchenko, with six aces, doubled Williams' total.

Lepchenko also hit more winners, 24 to 14, but was done in primarily by making more unforced errors, 32 to 21.

As for Lepchenko's serve effectiveness, Williams said: "I think when someone is a lefty, they just open the court more. You're expecting one serve. You almost give up one side. You just say, 'You can ace me on this side, but I'm going to expect that side.' "

Another potential semifinal opponent for Williams is Krunic, an Energizer Bunny player who scampers around the court and does the split slides so painfully reminiscent of Kim Clijsters. The 21-year-old Serbian was just too much for Kvitova, who never seems to do well here. Krunic surprised herself by beating her, which followed an equally surprising win over American Madison Keys.

"No, honestly," she said when asked if she thought she could beat Kvitova. "Also against Madison. When you are playing smaller tournaments all the time, I honestly don't know what to expect myself at all. I don't know my limits. Today I pushed myself to my total limits."

She will get pushed even further in her next match when she faces Azarenka.

Williams has pushed the outer limits of tennis since she won her first Grand Slam at the Open in 1999.

Despite a mediocre major season, she's going into the second week here with a clear mission and an acerbic wit.